Paper machine felts of a copolymer of 1,4-dimethylolcyclohexane, terephthalic acid, and isophthalic acid

ABSTRACT

This invention relates to a paper machine clothing having improved hydrolysis resistance and suitable for use in the forming, pressing or drying sections of a paper making machine, and has particular reference to paper making machine clothing used in the dryer section of a paper making machine, such as through air drying fabrics, and dryer screens. 
     According to one aspect of the present invention, there is provided an article of paper making machine clothing suitable for use in the forming, pressing or drying sections of a paper making machine which article includes a fibre structure characterized in that the fibres of said structure comprise a polyester material having a hintered carboxyl group and in that said fibres have a melting point greater than 260° C. The fibres are a copolymer of terephthalic acid, 1,4-dimethylolcyclohexane and isophthalic acid.

DESCRIPTION

This invention relates to paper machine clothing suitable for use in theforming, pressing or drying sections of a paper making machine and hasparticular reference to paper making machine clothing used in the dryersection of a paper making machine, such as through air drying fabrics,and dryer screens.

In paper making machines, a slurry of paper making constituents referredto as "furnish" is deposited on a fabric or "wire" and the liquidconstituent of the furnish is drawn or extracted through the fabric orwire to produce a self-cohesive sheet. This cohesive sheet is passed toa pressing and drying section of a paper making machine. In the pressingsection of the machine, the paper sheet is transported by a felt to apair of rollers where the felt and paper sheet are passed between thenip of the rollers to dewater and dry the paper sheet. The paper sheetitself may contain all types of chemical finishes and will be at thesame time, subjected to an elevated temperature in order to aid thedewatering and drying thereof.

After pressing the paper sheet passes to the drying section of themachine where it is dried at an elevated temperature. The fabric in thedrying section of the machine together with its sheet of paper tends tobe subjected to elevated temperatures in a rigorous chemicalenvironment. Dryer fabrics or "dryer screens" employed in the papermaking industry have, traditionally, been formed from a variety ofmaterials such as poly(ethylene terephthalate), polyphenylene sulfideand polypropylene. Each material has different properties and pricing,which affects its relative position in the marketplace. An importantproperty for any material used as a drier screen in a paper makingmachine is that the material should have good hydrolytic stability andgood dimensional stability.

Polypropylene is the cheapest material presently available; it hasexcellent hydrolytic stability, but poor dimensional stability atelevated temperature, and as a result it has only limited use.

Poly(ethylene terephthalate) (PET) is moderately priced, has exceptionaldimensional stability and reasonable hydrolytic stability. Poly(ethyleneterephthalate) is the predominant material currently used in themarketplace and in most cases, the hydrolytic stability of poly(ethyleneterephthalate) can be improved by the addition of carbodiimidestabilisers. Polyphenylene sulfide has excellent dimensional andhydrolytic stability, but suffers from the disadvantage that it isextremely highly priced, is more difficult to work, and tends to sufferfrom brittle fracture problems in the crystalline state due to normalflexing experienced on the paper machine.

According to one aspect of the present invention, there is provided anarticle of paper making machine clothing suitable for use in theforming, pressing or drying sections of a paper making machine whicharticle includes a fibre structure characterised in that the fibres ofsaid structure comprise a polyester material having a hintered carboxylgroup and in that said fibres have a melting point greater than 260° C.The fibres may have a creep extension of less than 10% at 1.1 grams perdenier.

For the purposes of this specification fibre refers to a shapedpolymeric body of high aspect ratio capable of being formed into two orthree dimensional articles as in woven or nonwoven fabrics. Fibrefurther refers to staple, multifilament or monofilament forms. Meltingpoint is defined in this context as the temperature of the highest peakon the endotherm of the plot produced via Differential ScanningCalorimetry. By way of example of how melting point is determined FIG. 1(hereinafter referred to) is a graph of a Differential ScanningCalorimetry response of a commercial polyester with a melting point of255° C.

In another aspect of the present invention, the fibres may additionallyhave an initial modulus greater than 25 grams per denier, an elongationat break of greater than 15% and a tenacity of greater than 2 grams perdenier.

In a further aspect of the present invention the fibres may have amelting point greater than 265° C. and an initial modulus greater than30 grams per denier and an elongation at break of greater than 25%, atenacity of 2.2 grams per denier.

A further embodiment of the present invention provides that the fibreshave a melting point of greater than 280° C. and an initial modulusgreater than 32 grams per denier, an elongation at break greater than30%, a tenacity of greater than 2.3 grams per denier and a creepextension of less than 8% at 1.5 gram per denier.

A further aspect of the present invention provides that the polyestermaterial has carboxyl groups which are hindered by a moiety selectedfrom cyclicaliphatic and branched aliphatic glycol. The polyester may bepoly(1,4-cyclohexandicarbinyl terephthalate). In this polymer, thecyclohexane ring may be substituted such that the two carbinyl groupsmay exist in one of two configurations, i.e. the cis- or thetrans-configuration. While the precise mechanism is not entirelyunderstood, the cis-configuration imparts a relatively low melting pointof the order of 220° C. while the trans-configuration has a high meltingpoint approaching 300° C. and is highly crystalline.

The large size of the cyclohexane moiety within the polyester moleculeserves to hinder a hydrolytic attack on the carboxyl group and isthought to provide improved hydrolysis resistance. At the same time, thethermal properties of the material can be controlled by selection of therelative proportions of the cis- and trans-isomers to produce a materialwhich is eminently suitable for use in high temperature portions of apaper making machine such, for example, as a dryer screen.

The polyester material may include a proportion of a stabiliser. Typicalstabilisers include carbodiimides present in an amount of 0.5 to 10%,preferably 1 to 4% by weight. The carbodiimide may be that ofbenzene-2,4-diisocyonato-1,3,5-tris(1-methylethyl) homopolymer or it maybe that of a copolymer of 2,4-diisocyanato-1,3,5-tris(1-methylethyl)with 2,6-diisopropyl dissocyanate such, for example, as thatcommercially available under the trade name "STABAXOL P" or "STABAXOLP-100", respectively of Rhein-Chemie, of Rheinau GmbH, West Germany.

The polyester fibres either alone or incorporating the stabilisertypically have a tensile strength of 2.4 to 4.3 grams per denier. Thefibres of the fibre structure in accordance with the present inventionmay further exhibit a thermal shrinkage at 200° C. of 0.2% to 20.5% witha tensile modulus within the range of 34 to 74 grams per denier. In aparticular embodiment of the present invention, the polyester materialmay be poly(1,4-cyclohexanedicarbinyl terephthalate) and it has beenfound that the material commercially available under the trade name"KODAR THERMX copolyester 6761" (generally, a copolymer comprised ofterephthalic acid, 1,4-dimethylolcyclohexane and isophthalic acid.)produced by the Eastman Chemical Products Inc., is particularly suitablein this regard.

As stated above, one of the more important features of paper machineclothing in accordance with the present invention is its potential usein high temperature sections of a paper making machine, in particulardryer fabrics and dryer screen fabrics, since the material from which itis made is not readily hydrolyzed. Unexpectedly, materials in accordancewith the present invention show an exceptional degree of stability overtime when compared with conventional polyester materials currentlyemployed and it is not uncommon for the half life of the percentretained tensile strength for articles of paper machine clothing inaccordance with the present invention to be 1.5 to twice that of thecurrent industry standard.

While the invention is particularly concerned with materials suitablefor use in the drying section of a paper making machine, it will beappreciated by the person skilled in the art that with the tendencytowards ever higher temperatures in the forming and pressing sections ofa paper making machine, articles of paper making clothing in accordancewith the present invention can well be produced for use in both thepressing section and the forming section. In the forming section it ispossible to form an open weave using monofilament materials which allowfor adequate support of the solid materials in the furnish and yet allowsufficient dewatering to produce a coherent sheet preparatory topressing. In the pressing section, by providing both the support layerand at least a proportion of the surface layer of the pressing fabric inaccordance with the present invention, pressing fabrics much moretolerant of high temperature operation are produced.

The invention, therefore, is concerned not only with the production ofpaper machine clothing (PMC) materials which may be of woven or spiralor of other suitable monofilament structures, in which monofilaments mayextend in both the machine direction and the cross direction of thefabric, but also include other PMC structures. Such polyester may beused to produce PMC fabrics comprised of staple, multifilament, and/ormonofilament fibres.

Typical range of sizes of monofilaments used in Press Fabrics and DryerFabrics are 0.20 mm-1.27 mm in diameter or the equivalent mass incross-section in other cross-section shapes, e.g. square or oval.

For forming fabrics finer monofilaments are used, e.g. as small as 0.05mm. While special Industrial applications may use monofilaments up to3.8 mm.

Following is a description by way of example only and with reference tothe accompanying drawing of methods of carrying the invention intoeffect.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a graph of a differential scanning calorimetry response of acommercial polyester sample having a melting point of 255° C.

FIG. 2 is a graph showing the variation of hydrolysis resistance againsttime for various samples.

FIG. 3 is a plot of retained tensile stregth of a polyester sample withtime in an autoclave as set out in Example 7.

FIG. 4 is a plot similar to FIG. 3 for the sample of Example 8.

EXAMPLE 1

A polyester commercially available under the trade name "KODAR THERMXcopolyester 6761" (generally, a copolymer comprised of terephthalicacid, 1,4-dimethylolcyclohexane and isophthalic acid.) supplied by theEastman Chemical Products Inc. was extruded in a 25 mm single screwextruder having a screw with a compression ratio of 4.12 and a 40 meshscreen filtration at the end of the barrel. The material was spun afterfiltration through a 325 mesh screen supported by an 80 mesh screenthrough a multi-hole die with each hole having a diameter of 0.625 mm(0.025"), land length of 1.9 mm. The air gap after extrusion was 32 mmandthe quench water temperature was 66° C. The resultant extrudate wassubjected to an overall draw ratio which varied from 3.0 to 4.8 therebyproducing a range of denier of the monofilaments.

                                      TABLE 1                                     __________________________________________________________________________    UNSTABILIZED FIBER PROPERTIES                                                                               ELONGATION                                                                             INITIAL                                SAMPLE AVERAGE                                                                              OVERALL  TENACITY                                                                             AT BREAK MODULUS                                (Al NB No.)                                                                          DENIER DRAW RATIO                                                                             (gpd)  (%)      (%)                                    __________________________________________________________________________    3458-63-1                                                                            393    4.4      3.7    12       63                                     3458-63-2                                                                            371    4.8      4.5     8       80                                     3458-64-1                                                                            388    4.4      3.7     7       79                                     3458-64-2                                                                            506    3.4      2.6    26       55                                     3458-65-1                                                                            560    3.0      2.5    38       43                                     3458-65-2                                                                            424    4.0      3.7    18       59                                     3458-65-3                                                                            422    4.0      3.6    16       57                                     __________________________________________________________________________

EXAMPLE 2

The experiment as defined in Example 1 was repeated for a proportion ofthesame copolyester material having various proportions of up to 5% byweight of a carbodiimide stabilizer material commercially availableunder the trade name "STABAXOL P-100". The properties of themonofilament as extruded and drawn are set out in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    STABILIZED FIBER PROPERTIES                                                                 STABILIZER     ELONGATION                                                                             INITIAL                                 SAMPLE AVERAGE                                                                              CONTENT TENACITY                                                                             AT BREAK MODULUS                                 (Al NB No)                                                                           DENIER (%)     (gpd)  (%)      (gpd)                                   __________________________________________________________________________    3458-90-1                                                                            432    5.0     3.5    18       53                                      3458-91-4                                                                            431    3.0     3.5    18       53                                      3458-91-9                                                                            430    1.5     3.6    18       53                                      __________________________________________________________________________    NOTE-OVERALL DRAW RATIO FOR ALL SAMPLES IS 4.0                            

FIG. 2 shows graphically how the hydrolysis resistance of the variousstabilized and unstabilized monofilaments described in Examples 1 and 2behave over a period of 32 days when subjected to saturated steam in anautoclave at a pressure of 2 atm absolute pressure. The five samples ofTable 2 are illustrated together with a commercial monofilament producedfrom poly(ethylene terephthalate) and stabilized with a cabodiimide. Thesignificant point on the graph is the period in which the retainedtensilestrength has been reduced to 50%.

From FIG. 2 it will be seen that the three samples which had thecarbodiimide stabiliser present, retained their tensile strength over alonger period, in some cases more than double that of the other threesamples which did not contain stabiliser. And in all samples, bothstabilized and unstabilized, hydrolysis resistance was superior to thatofconventional poly(ethylene terephthalate) stabilized with acarbodiimide.

Sample fabrics of extruded material were formed into dryer screenfabrics by weaving the monofilament in both the machine andcross-machine directions. The fabrics were run in a dryer sectionvis-a-vis presently used fabrics of poly(ethylene terephthalate), bothalone and with stabilisers. It was found that the life of the fabrics inaccordance with the present invention, showed a significant increaseover those manufactured from traditional materials such as poly(ethyleneterephthalate).

EXAMPLE 3

"KODAK THERMX copolyester 6761" (generally, a copolymer comprised ofterephthalic acid, 1,4-dimethylolcyclohexane and isophthalic acid.) wasfed to a 25 mm extruder having a single flighted screw having acompression ratio of 4.12. A metering pump was attached to the extruderand used to meter polymer to a spin pack. The spin pack containedfilters which were comprised of a 400 mesh screen supported by a 200mesh screen, which was supported by an 80 mesh screen. The spin packalso contained a die having 8 holes each hole having a diameter of 1.3mm. Polymer was extruded vertically from the die into a water quenchbath. The air gap between the die face and quench bath was 32 mm. Thequench bath temperature was 66° C.

The extruded filament travelled through the bath for an approximatequench length of 0.8 mm. The filament exited the bath horizontally andtravelled to a first roll stand operating at a speed of 8 m/min. Thefilament then passed through a hot air circulating oven operating at121° C. The oven was 1.6 meters long. The filament exited the oven andtravelled to a second roll stand operating at 28 m/min. The filamentthen passed through a second oven operating at 149° C. and travelled toa third roll stand operating at 39 m/min. The second oven had a lengthof 1.6 meters. The filament then passed through a third oven operatingat 177° F. and passed to fourth roll stand operating at a speed of 32m/min. The third oven had a length of 1.6 meters. The orientedmonofilament was then collected on a spool via a tension controlledwinder. The product when tested had a tensile strength of 3.4 gpd, anelongation at break of 23.5%,an initial tensile modulus of 41.0 gpd anda thermal free shrinkage at 200° C. of 7.6%.

EXAMPLE 4

This Example is similar to Example 3 with the following changes in rollstand speeds. The speeds for the first, second, third and fourth rollstands were 8, 28, 28 and 25 m/min, respectively. The product whichresulted had a tensile strength of 2.7 gpd, an elongation at break of34.8%, an initial tensile modulus of 36.3 gpd and a thermal freeshrinkageat 200° C. of 4.6%.

EXAMPLE 5

This Example is similar to Examples 3 and 4, equipment wise, but withchanges in both oven temperatures and roll stand speeds. The oventemperatures were 177°, 204° and 500° for ovens one, two and three,respectively. The speeds for the first, second, third and fourth rollstands were 8, 36, 39 and 39 m/min, respectively. The product whichresulted had a tensile strength of 4.6 gpd, an elongation at break of7.4%, an initial tensile modulus of 74.4 gpd and a thermal freeshrinkage at 200° C. of 11.6%.

EXAMPLE 6

This Example is similar to Example 5 with the following changes in rollstand speeds. The speeds for the first, second, third and fourth rollstands were 8, 32, 32 and 32 m/min, respectively. The product whichresulted had a tensile strength of 4.0 gpd, an elongation at break of18.0%, an initial tensile modulus of 55.3 gpd and a thermal freeshrinkageat 200° C. of 5.9%.

EXAMPLE 7

"KODAR THERMX copolyester 6761" (generally, a copolymer comprised ofterephthalic acid, 1,4-dimethylolcyclohexane and isophthalic acid.) and"STABAXOL P" at a concentration of 2.2% was fed to a 50 mm extruderhavinga single barrier flighted screw having a compression ratio of 3.1.A metering pump was attached to the extruder and used to meter polymerto a spin pack. The spin pack contained filters which were comprised ofa 180 mesh screen supported by a 250 mesh screen, which was supported bya 60 mesh screen. The spin pack also contained a die having 10 holeseach having a diameter of 1.5 mm. Polymer was extruded vertically fromthe die into a water quench bath. The air gap between the die gace andthe quench bath was 30 mm. The quench bath temperature was 66° C. Theextrudedfilament exited the bath horizontally and travelled to a firstroll stand operating at a speed of 20 m/min. The filament then passedthrough a hot air circulating oven operating at 121° C. The oven was 2.7meters long. The filament exited the oven and trvelled to a second rollstand operating at 69 m/min. The filament then passed through a secondoven operating at 191° C. and travelled to a third roll stand operatingat 70 m/min. The second oven had a length of 2.4 meters. The filamentthenpassed through a third oven operating at 268° C. and passed to afourth roll stand operating at a speed of 62 m/min. The third oven had alength of 2.7 meters. The oriented monofilament was then collected on aspool via a tension controlled winder. The product when tested had atensile strength of 2.5 gpd, an elongation at break of 33%, and aninitialmodulus of 32 gpd.

FIG. 3 shows graphically how the hydrolytic resistance of the stabilizedmonofilament described in Example 7 behaves over a period of 38 dayswhen subjected to saturated steam in an autoclave at a pressure of 2 atmabsolute pressure.

EXAMPLE 8

"KODAR THERMX copolyester 6761" (generally, a copolymer comprised ofterephthalic acid, 1,4-dimethylolcyclohexane and isophthalic acid.) and"STABAXOL P" at a concentration of 2.5% was fed to a 70 mm extruderhavinga single barrier flighted screw having a compression ratio of 2.5.A metering pump was attached to the extruder and used to meter polymerto a spin pack. The spin pack contained filters which were comprised ofa 180 mesh screen supported by a 250 mesh screen, which was upported bya 60 mesh screen. The spin pack also contained a die having 50 holeseach having a diameter of 1.5 mm. Polymer was extruded vertically fromthe die into a water quench bath. The air gap between the die face andthe quench bath was 57 mm. The quench bath temperature was 63° C. Theextrudedfilament exited the bath horizontally and travelled to a firstroll stand operating at a speed of 17 m/min. The filament then passedthrough a hot air circulating oven at 179° C. The oven was 2.7 meterslong. The filament exited the oven and travelled to a second roll standoperating at58 m/min. The filament then passed through a second ovenoperating at 231° C. and travelled to a third roll stand operating at 58m/min. The second oven had a length of 2.7 meters. The filament thenpassed through a third oven operating at 257° C. and passed to a fourthroll stand operating at a speed of 52 m/min. The third oven had a lengthof 2.7 meters. The oriented monofilament was then collected on a spoolviaa tension controlled winder. The product when tested had a tensilestrengthof 2.6 gpd, an elongation at break of 39%, and an initialmodulus of 32 gpd.

FIG. 4 shows graphically how the hydrolytic resistance of the stabilizedmonofilament described in Example 8 behaves over a period of 38 dayswhen subjected to saturated steam in an autoclave at a pressure of 2 atmabsolute pressure.

We claim:
 1. An article of paper machine clothing used in the forming,pressing or drying sections of a papermaking machine, which articleincludes a fibre structure characterized in that the fibres of saidstructure consist essentially of a woven polyester material which is acopolymer of terephthalic acid, 1,4-dimethylolcyclohexane andisophthalic acid the polyester material exhibiting an increase in lifeof the clothing relative to clothing comprised of polyethyleneterephthalate, and said fibres have a melting point greater than 260° C.2. An article as claimed in claim 1 wherein the fibers are characterizedby a creep extension of less than 10% at 1.1 gpd.
 3. An article asclaimed in claim 1 further characterised in that the fibres have aninitial modulus greater than 25 gpd, an elongation at break of greaterthan 15%, and a tenacity greater than 2 gpd.
 4. An article as claimed inclaim 1 characterised in that said fibres have a melting point greaterthan 265° C., an initial modulus greater than 30 gpd, an elongation atbreak greater than 25%, a tenacity of 2.2 gpd.
 5. An article as claimedin claim 1 characterised in that said fibres have a melting pointgreater than 280° C., an initial modulus greater than 32 gpd, anelongation at break greater than 30%, a tenacity greater than 2.3 gpd.6. An article as claimed in claim 1 characterised in that the polyestermaterial includes an effective amount of a stabiliser.
 7. An article asclaimed in claim 6 characterised in that the stabiliser is present in anamount of 0.5% to 10.0% by weight.
 8. An article as claimed in claim 6characterised in that the stabiliser is a carbodiimide.
 9. An article asclaimed in claim 8 characterised in that the carbodiimide isbenzene-2,4-diisocyanate-1,3,5-tris(1-methylethyl) homopolymer.
 10. Anarticle as claimed in claim 8 characterised in that the carbodiimide isa copolymer of benzene 2,4-diisocyanato-1,3,5-tris(1-methylethyl) and2,6-diisopropyl diisocyanate.
 11. An article as claimed in claim 1characterised in that the fibre is a monofilament of either round orother shaped cross-sections.
 12. An article as claimed in claim 11 inwhich said fibres are monofilaments extending in the machine direction.13. An article as claimed in claim 11 in which said fibres aremonofilaments extending in the cross machine direction.
 14. An articleof paper machine clothing used in the forming, pressing or dryingsections of a papermaking machine, which article includes a fibrestructure characterized in that the fibres of said structure consistessentially of a woven polyester material which is a copolymer ofterephthalic acid, 1,4-dimethylolcyclohexane and isophthalic acid thepolyester fiber material being selected on the basis of its ability tobe woven into a papermachine clothing, said fibres have a melting pointgreater than 260° C.
 15. An article of paper machine clothing used inthe forming, pressing or drying sections of a papermaking machine, whicharticle includes a fibre structure characterized in that the fibres ofsaid structure consist essentially of a woven polyester material whichis a copolymer of terephthalic acid, 1,4-dimethylolcyclohexane andisophthalic acid the clothing made of said polyester material exhibitingimproved wear resistance and duration of use relative to paper machineclothing made of polyethylene terephthalate, and said fibres have amelting point greater than 260° C.